1. Field of the Invention
This invention relates to thickness measurement devices, and particularly to the employment of FM radar for the measurement of coal thickness.
2. General Description of the Prior Art
Frequency modulated radar systems have been employed to measure the thickness of coal, that is, the distance between the face of a seam of coal back to a shale or other backing of the coal. Range is measured as a frequency function, an increase in frequency of the detected signal indicating an increase in thickness. Readout of thickness is commonly accomplished by observing amplitude peaks of the signal on a spectrum analyzer. Peak maximums are denoted on the display as a series of line spectrum components spaced at the modulating rate of the radar, typically 100 Hz. The distance from the radar antenna to a target, or interface of the coal with another medium, is apparent by a series of spectral lines increasing in amplitude and reaching a peak at a frequency corresponding to the distance to the interface. In the case of coal thickness, two or more peaks will be observed corresponding to each of the front and back surfaces of the coal.
This invention deals with two of the problems of radar systems used for this type of measurement. One involves spectrum splitting which occurs when the round-trip distance between the radar antenna and target, coal interface, is an odd multiple of equivalent line spectrum spacings. For example, an FM-CW radar operating over a frequency range of 1.7 to 4.3 GHz with triangular modulation will have a range sensitivity of approximately 35 Hz per cm. In such case, if the radar video spectrum is examined, a line every 100 Hz will be found with a maximum amplitude corresponding to a target (coal interface) range. As an example, a target at a 30 centimeter range might produce a peak in energy at 4 KHz. However, if the target to radar distance is changed by a small amount (approximately 1.2 cm), the maximum energy peak that was present before this small change in distance now becomes a minimum. This spectrum splitting produces two apparent targets and reduces operator ability to determine target range to an accuracy better than 100 Hz, 2.6 cm or 1.1 inch. A further problem in measuring thickness by radar, particularly with FM-CW type systems, is the extremely large echo from the front coal surface, which echo overlaps or masks the very weak echo from the distant target (coal-shale interface). In fact, when the thickness of the coal is greater than a few inches, it becomes difficult to identify the true location of the coal-shale interface.
It is the object of this invention to overcome the aforesaid and other difficulties.